This invention relates to command and ranging systems, and more particularly to a method and apparatus for multiple rate digital command detection with range cleanup capability upon transmitting commands to a spacecraft on the ranging subcarrier.
Most spacecraft employ phase-locked receivers that track an unmodulated (or residual) uplink carrier signal. Consequently, all information transmitted to the spacecraft must be such that, when modulated onto the carrier signal, the information contained in the modulation sidebands falls outside the receiver tracking loop bandwidth. Because of this, information signals containing a large amount of low-frequency energy (such as command or ranging signals) must first be modulated onto an intermediate carrier (or subcarrier) before finally being modulated onto the carrier. The introduction of the subcarrier, however, is not a "cure-all" since in order to coherently demodulate the information signal one must coherently track the subcarrier in addition to the carrier and data. The standard approach is to provide independent tracking loops to "track-out" all of the information contained in each of the carriers.
In the past a dedicated unmodulated carrier uplink signal has been required for a spacecraft narrow-band, phase-locked-loop receiver to track signals from the ground. Low frequency signals, such as commands to the spacecraft, have been required to fall outside the tracking loop bandwidth and have been first modulated onto an intermediate carrier or subcarrier before being modulated onto the carrier. This required that the spacecraft provide independent tracking loops to demodulate all of the information on these carriers and subcarriers. Additionally, a separate subcarrier channel was required for ranging.
In respect to ranging information, binary coded, sequential acquisition ranging systems of the so-called .mu.-type have been used in an arrangement disclosed in the U.S. Pat. No. 3,659,292. Ranging systems of that type are improved by a code regenerative cleanup loop disclosed in U.S. Pat. No. 3,729,736. The cleanup loop technique is to track and detect in sequence binary codes phase modulated on a subcarrier where the range code period is successively increased for greater range resolution. In practice, the subcarrier and modulating range code bit rates are related by a power of two, but such a restraint is not necessary.